In an AMPS communications system FM (frequency modulation) is used for communicating voice signals with a frequency in the range from 300 to 3400 Hz, a supervisory audio tone (SAT) with a nominal frequency selected from three possible nominal frequencies of 5970, 6000, and 6030 Hz, and a signalling tone (ST) with a nominal frequency of 10 kHz. The ST, which has a nominal FM peak deviation (range of frequency change) of 8 kHz, is used by mobile (or fixed) terminals to communicate with base stations in accordance with an AMPS protocol that assigns different burst durations of the ST to different events. For example, ST burst durations of 50 ms, 400 ms, and 1800 ms correspond respectively to a hand-off acknowledgement, a request to send dialled digits, and a release. In order to detect such bursts, the ST detector must have an attack time (time from the start of a burst to report the presence of ST) of 30 ms or less and a release time (time from the end of a burst to report the loss of ST) of 300 ms or less. These requirements must be met whether or not the SAT is present at any of its frequencies.
Different terminals may produce the ST with considerable variations, from their nominal values, of ST frequency and peak deviation. More specifically, different terminals may produce the ST with a frequency anywhere in a range of 9.5 to 10.5 kHz, and with a peak deviation anywhere in a range of 6 to 10 kHz. An ST detector in a base station is required to detect the ST regardless of these variations.
Furthermore, the communications channel is subject to co-channel interference (CCI), and both the CCI and desired signals are subject to fading. The output of the demodulator of the base station, to which the ST detector is responsive, consequently has highly variable signal and noise (including interference) powers, and can contain high-amplitude clicks. Reliable detection of the ST is still required in these circumstances.
It is known to use a PLL (phase-locked loop) for detecting a tone of known frequency, but this is not generally satisfactory for AMPS ST detection because the input additive noise is not white Gaussian, the ST can be considerably offset from its nominal frequency, and a short attack time is required of the ST detector in order to detect relatively short ST bursts.
Miller-Thomson et al. U.S. Pat. No. 4,546,490 issued Oct. 8, 1995 entitled "Single Frequency Detection Circuit" describes a detector for detecting, in the presence of voice signals at a lower level by at least 10 dB, the SF (single frequency) 2600 Hz in-band signalling tone in telephone equipment. That detector rectifies and filters the incoming signal to produce a signal without the fundamental component of any SF tone which may be present, low pass filters the signal and determines a magnitude of any SF tone from the resulting d.c. component, and also high pass filters the signal and further rectifies and filters the result to determine a magnitude of the voice signal, comparing the magnitudes to determine the presence or absence of the SF tone. Such a detector can take advantage of the 10 dB or more difference between levels of any SF tone and voice signals, and is not required to accommodate the fading and CCI to which AMPS signals are subject. In addition, the design of the filters in such a detector can rely on the relatively small (.+-.38 Hz, or less than 1.5%) possible variation in the frequency of the SF tone, in comparison to the relatively wider (.+-.0.5 kHz, or 5%) possible variation in the frequency of the AMPS ST.
An object of this invention is to provide an improved tone detector and method of tone detection, which can facilitate ST detection in an AMPS communications system.